Clarify how topics.py gets created. (#106121)

When changing docs, it was easy to find text in topics.py, and I
wondered whether I was supposed to edit it.  Thankfully, the top of the
file says it's auto-generated, so I knew I didn't have to edit it. But I
didn't know what started the auto-generation process.

It's part of the release process, so I'll leave a note here for future
editors.